Electron diffraction studies show that a 2ao x 2co super1attice
forms in TiSe2 at low temperature. Strong diffuse scattering is observed above T. An optically active far-infrared lattice mode is observed which
c breaks up into several additional modes below the phase transition. In the infrared study, there is a dramatic shift in the Drude edge as well as new absorption peaks at low temperature. These observed phenomena are related to the energy gap which opens up in the band structure below the phase transition temperature. Strong diffuse scattering, which persists down to lOOK, is seen in TiS and HfTe2. No structural transition is observed in these crystals,
2 2but the diffuse scattering may be a precursor to super1attice formation.